Local folder backups

Having just returned to using an email client (used webmail for a while) i'd be grateful if someone could suggest how often one should backup the files. Also having read a couple of posts on losing messages [usually due to inbox getting too large] - what is the SAFE limit to go to. I remember my inbox in Eudora crashing several years ago.

Sorry if this is listed in their help file but i couldn't see it.

thanks